Dissipative load



B. W. GRIFFITH, JR

Oct. 8, 1957 DIS'SIPATIVE LOAD Filed Oct. 9. 1953' l IIIIIII|ININVENTOR. 5. me/a Gr/Yfifh, J1:

ATTORNEYS United States Patent nice 2,809,355 Patented Oct. 8, 1957DISSIPATIVE LOAD Benjamin Whitfield Grifiith, Jr., Dallas, Tex.,assignor to Continental Electronics Manufacturing Company, Dallas, Tex acorporation of Texas Application October 9, 1953, Serial No. 385,971

6 Claims; (Cl. 333-22) The present invention relates to dissipatingtransmission lines to dissipate radio frequency energy, and moreparticularly to dissipative transmission lines of high power handlingcapacity.

In the design of dissipative transmission lines, it is often necessaryto effect a compromise between power handling capacity, voltage handlingcapacity, and physical size of the line. Decreasing the spacing betweenconductors decreases the characteristic impedance and the sending endimpedance of the line and, with a given materialto dissipate energy,increases the attenuation of the line. However, the possibility ofvoltage breakdown between the conductors of the line is increased, andtherefore the voltage capacity of the line decreased. If the voltagehandling capacity is important, the line may have to be extended toundesirable lengths to obtain sufiicient power handling capacity.

It is accordingly an object of the present invention to decrease thephysical size of dissipative transmission lines having a certain powerand voltage handling capacity.

it is another object of the present invention to increase the voltagehandling capacity of dissipative transmission lines without increasingthe size thereof.

With the above objects in view, the invention relates to dissipativetransmission lines, adapted to be connected to a source of radiofrequency energy, which are characterized by a pair of substantiallysimilar transmission lines of equal length, each having one end adaptedto be connected to the source of energy so that they are connected inparallel; the other end of one of the lines of the pair is shortcircuited, while the other end of the other line (of the pair) is opencircuited. The result of this connection is that the sending endimpedance for a thus connected line (one open, the other shortcircuited) will be identical with the sending end impedance of a linewhich is short circuited at the far end, and which is twice as long aseither of the pair of lines.

It is to be noted that the total length of line, or wire, necessary isnot changed. However, when the one-open, one-short circuited method ofconnection of two lines in parallel is being used, each of the pair oflines may have twice the characteristic impedance as the characteristicimpedance of a single, long line when a given sending end impedance isdesired; this, in turn permits greater spacing between conductors, andtherefore improves the voltage handling capacity. Further, it is ofteneasier to physically arrange two lines next to each other rather than asa single, long line.

The invention is not limited to any particular kind of dissipativetransmission line; it might be an open wire line consisting of stainlesssteel wire, a coaxial line containing a purely resistive element as thecentral conductor, or it might be constructed with a resistive elementcomprising a large number of finely dispersed ferromagnetic particlessurrounding a conductor, such as more fully described and claimed in mycopending application Serial No. 386,034 filed Oct. 14, 1953.

In the accompanying drawing I have illustrated two examples ofconnecting the pair of lines to a source of energy.

Figure l is a schematic diagram of a pair of dissipative lines connectedaccording to the invention.

Figure 2 is a schematic diagram of a balanced pair of co-axialdissipative lines connected to the source of R.-F. energy.

Referring now to the drawings, and more particularly to Fig. l, 10schematically designates a source of R. F; energy having output leads 11and 12. The dissipative transmission line is shown, for purposes ofillustration, as an open wire line, for example of steel wires composingcomponent lines A and B, each line consisting of wires 13 and 14, and 15and 16. Wires 13 and 14 form together one of the pair of dissipativetransmission lines, and wires 15 and 16 form the other one of the pair.Wires 13 and 14 (component line A), as well as wires 15 and 16(component line B) are connected to leads 11 and 12 at one of theirends. At the other end, the component line A (wires 13, 14) is shortcircuited, as shown at 17. The other end of component line B (wires 15and 16) is open circuited as shown at 18.

Fig. 2 illustrates a pair of co-axial dissipative lines connected to aload so that the connection is balanced with respect to ground. Thesource of R. F. energy, 10, is connected to a center tapped transformer,20; the center tap is connected to ground in a manner well known in theart. One side each of the transformer 20 is connected to the centerconductors 21, 22, 23 and 24 of co -axial dissipative transmission linecomponents C, D, E and F, the outside conductors 25, 26, 27 and 28 beinggrounded as shown in the drawing. The center conductor preferably is aresistive element, for example, a carbon rod. The four line componentsC, D, E and F should be as alike to each other as possible, to obtain agood balance. Two lines, C and D, and E and F, respectively, form a pairconnected in parallel. One line of the pair, here shown to be lines Cand E are short circuited at the end removed from the connection to thepower source; the other line, of each pair, here shown as lines D and F,is open circuited.

It is of course obvious that the co-axial lines may also be replaced byopen Wire dissipative lines, or any other dissipative line; theparticular type of dissipative line is of no consequence, and theoperation of the system is independent of the type of line.

MATHEMATICAL CONSIDERATIONS Definition of symbols used:

Zocharacteristic impedance of one component of line (A or B, Fig. l)

Zssending-end impedance of short circuited component of lineZotsending-end impedance of open circuited component of lineYocharacteristic admittance of one component of line Ys..sending endadmittance of short circuited component of line Yo .sending endadmittance of open circuited component of line L-length of each lineapropagation function of each line, equal to oL +jll Yadmittance atjunction of two transmission lines, the sending end terminals of whichare connected in parallel Zimpedance at junction of two transmissionlines, the sending end terminals of which are connected in parallelSending end impedance of a single short circuited transmission line:

Zsc=Zo tanhaL Sending end impedance for a single open circuited line:

Zoc=Zo COthaL Sending end admittance of single, short circuited line:

Connecting two lines, one open circuited, and one short circuited, inparallel, the sending end admittance will be:

Y: Ysc+ Yoc Y: Yo COlIhaL-l- Yo taDhoLL V Y: Yo (COtllccL +taI1l1ocL)Since: coth+tanh6=2 coth 20; and substituting: Y=2Yo coth 2ozL; then thesending end impedance will be in comparison, two short circuited linesin parallel have a sending end impedance of /2Zsc= /2Z0 tanhaL.

it is therefore seen that the sending end impedance function for theline connected with one open circuited, and one short circuitedcomponent will be that of a transmission line twice as long having twoshort circuited components in parallel; in other words, the decrease inthe input impedance of the line (as seen from the sending end) byparallel connection is still obtained by the parallel connection of oneopen, and one short circuited line with the one open, one shortcircuited connection; however, the deviation of the sending endimpedance from the value of /2Z0, resulting from the non-infinite lengthof the line, will be much less where one line component is shortcircuited and one open circuited than where both line components areshort circuited or both open circuited.

I claim: 7

1. In a system of high frequency transmission lines, a broad frequencyband resistor comprising a pair of line components of substantiallyequal lengths, said pair being connected in parallel at one end thereof;one line component being open circuited at the other end, and the otherline component being short circuited at the other end, each of said linecomponents including a conductor formed of a resistive material.

2. A system according to claim 1, wherein the resistive material is astainless steel Wire forming one conductor of each line component.

3. A system according to claim 1, wherein each line component includes aconductor and finely dispersed ferromagnetic material surrounding saidconductor.

4. A system according to claim 3, wherein each line component is acoaxial line and said ferromagnetic material surrounds the innerconductor of said coaxial line.

5, A system according to claim 1, wherein each line component includes apair of conductors formed of a resistive material.

6. In a system of high frequency transmission lines, a broad frequencyband resistor comprising a pair of line components of substantiallyequal lengths, said pair being connected in parallel atone end thereof;one line component presenting at the parallel connected end an impedancecorresponding to open circuit condition, and the other line componentpresenting at said parallel connected end an impedance corresponding toshort circuit condition, each of said line components including energydissipating means for dissipating most of the energy supplied to saidline components.

References Cited in the file of this patent UNITED STATES PATENTS1,935,313 Feldrnan Nov. 14, 1933 1,939,053 Hunt Dec. 12, 1933 2,238,438Alford Apr. 16, 1941 2,532,993 Carter Dec. 5, 1950

